Why do I owe?

When you make more money you have to pay more tax. You might need to adjust your withholding. You can use the Tax Withholding Estimator on the IRS web site to figure out what to put on the W-4 forms that you give to your employers.

Did you get monthly advance payments of the Child Tax Credit? Those were advance payments of a credit that you get on your tax return. Since you got the money in advance, you get less when you file your tax return, or you might even owe. In effect, the advance Child Tax Credit payments were an early payment of your refund.

Hi

There can be several reasons.  But one reason is that by having 2 jobs from different employers, the 2 wages will push you into higher tax brackets.

I would guess that you claimed the same exemption amount for each employer.  When combining the 2 salaries, you probably should have had more withholding taken out of your paycheck(s) to cover the increased tax rate (brackets).

An extra $21K is quite a bit.  Without knowing a lot details, that would most likely be the reason.  Especially if you're single, no dependents, credits, etc.